1. Field of the Invention
The present invention pertains generally to the field of lighter-than-air airships. More particularly, the invention relates to the control and regulation of a lift gas such as helium along with its pressure within a tethered helium filled aerostat using a ground based lift gas ballast chamber. The invention includes a novel hollow tether with a lifting gas feed tube and novel double slip rings which allow weathervaning by providing a substantially airtight connection between the ground and the aerostat.
2. General Nature of the Prior Art
A tethered aerostat is an aerodynamic shaped, lighter-than-air vessel of a flexible structure filled with a lifting gas such as pressurized helium and mechanically anchored with a long high strength tether to a ground structure. An aerostat is equipped with a system of sensors, blowers and valves, which, in conjunction with a plurality of deformable air compartments called ballonets are used to control the pressure within the hull to maintain aerodynamic shape to minimize the drag force exerted on the airship by the ever present wind. To compensate for the diurnal and seasonal variation of ambient temperature as well as the solar gain during the daylight period, the volumes of air inside the ballonets are changed either by opening valves to allow air to be pushed out of the ballonets or by turning a blower on to blow outside air in to pressurize the ballonets to maintain pressure within the airship. The ballonets are also used to alter the aft-fore balance of the lift force in order to provide pitch control. The power needed on board the aerostat is delivered through the high voltage power cables embedded within the tether. The tether also contains one or more optical fibers to enable onboard equipment to communicate with the ground station.
3. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In the prior art due to inherent helium leakage and lift gas contamination due to an air infiltration, a tethered aerostat needs to be brought down periodically to refill the helium gas and to perform regular maintenance. A typical tethered blimp with a helium volume of between 50,000 cubic feet and 100,000 cubic feet can be expected to stay up for only about 10 to 20 days. The aerostat is retrieved from altitude by drawing the tether back through a mechanized winch. As the aerostat is lowered, the helium gas contracts owing to the increased ambient pressure at lower altitude. This is compensated by pumping air into the ballonets to maintain the aerodynamic shape. To launch or re-launch the aerostat, the tether is gradually released to allow the aerostat to ascend to altitude, the expanding helium gas forces air from the ballonets through the air valves. The valves are controlled to prevent the helium pressure from dropping below a threshold level.
In addition to helium leakage, there is also the issue of air infiltration which occurs at a slower rate. Since air can not provide lift, the air inside the helium bag needs to be purged periodically. This is accomplished when the aerostat is returned to earth for maintenance with a helium scrubber which removes the air from the helium. Although it is possible to include helium scrubbing equipment aloft such as described in Haunschild U.S. Pat. No. 5,090,637. However to remove the air while the airship is aloft for the purpose of extending the mission duration is highly unattractive due to the additional weight, power requirements, and complexity, hence helium scrubbing is usually done on the ground.
Haunschild U.S. Pat. No. 5,090,637 recognizes the importance of removal of oxygen, nitrogen and other gases from helium. Haunschild however does maintenance on the lift gas either in the airship or on the ground. The known prior art does not provide for the maintenance of the lift gas on the ground while the airship is deployed by providing the bidirectional transfer of the lift gas between the ground and the aerostat.
The relatively short mission duration and the significant downtime involved in retrieving and re-launching the aerostat, as well as the time it takes to replenish and clean the helium makes it unappealing to use a tethered aerostat for tasks that demand high availability such as weather monitoring and telecommunications. Frequent launch and retrieval also drastically increase the risk of damaging the envelope of the aerostat, thereby shortening the service life of the entire system.
The use of ballonet to maintain the excess pressure called super-pressure to combat the diurnal pressure fluctuation of the helium gas also produces some unwelcome side effects, the primary of which is the cyclic variation of the lift force resulting from the cyclic expansion and contraction of the helium volume. Since the aerostat is physically constrained by the tether, this leads to the cyclic variation of the tension on the tether embedded cables and optical communication fibers. Such cyclic variation progressively weakens the tether over time. In addition, the constant switching of the blower on or off also drastically shortens the service life of the blower. Malfunction of the blower can cause the aerostat to lose its pressurization resulting in damage to the aerostat by the wind aloft by a sudden increase in wind drag. Consequently, the blower needs to be maintained and/or replaced with high frequency that can be reduced by maintaining the pressure of the lift gas by a bidirectional transfer of lift gas between the airship and the ground. Furthermore, ballonets and the attendant blower equipment of the prior art increases the size, weight, and cost of the airship.
Some very low altitude tethered aerostat systems deployed at an altitude of up to 300 feet use a feed-tube that either is embedded within the tether or runs parallel to the tether to allow the refilling of the helium to be performed while the aerostat is aloft. This is accomplished by connecting the proximal end of the feed-tube to a bottle of compressed helium gas and opening the valve partially to send a burst of helium gas up the feed-tube to replenish the helium. This prior art does not provide a bidirectional flow of lift gas or do maintenance on the lift gas by scrubbing the lift gas. In addition such known prior art feed tubes which would have wall thickness sufficient to accommodate high pressure bursts of helium could not be used in airships deployed at 5,000 feet due to weight which would limit the altitude of the airship. Further the known prior art has not provided for a transfer of lift gas in a weathervaning airship through airtight slip rings.
In addition in the prior art the relatively high helium pressure used in very low altitude tethered aerostat forces the helium gas up with a flow velocity well in excess of 50 m/s for altitude of up to 300 ft. Such a high flow rate can rapidly heat up the feed-tube and could eventually damage the feed-tube if the flow rate is sustained for a long duration and damage the airtight connection of the slip ring to accommodate weathervaning.
In the prior art the feed tube has to be thick walled in order to withstand the pressure employed to force helium lift gas into the pressurized aerostat which makes the tether too heavy for higher altitude applications. Even at such a high flow rate, the refilling of the helium would still take a long time. At higher altitudes, the increased length of the feed-tube increases the flow resistance, which drastically reduces the flow speed, making manual control of the refilling process infeasible. The flow rate can be increased by opening the valve fully to increase the pressure head. However, this requires the wall of the feed-tube to be even further strengthened to enable it to resist the much higher helium pressure head. This dramatically increases the thickness of the feed-tube and since the total weight of the feed-tube is proportional to the thickness as well as the length of the tube, the concomitant increase in the weight of the feed-tube makes such a scheme impractical.
The prior art includes a variety of slip rings and flying sheaves to allow weathervaning of the airship during launch flying or docking. Phipps III, et al. U.S. Pat. No. 4,402,479 provides slip rings for electrical cables and a flying sheave to accommodate weathervaning. Czarnecki, et al. U.S. Pat. No. 4,675,030 includes electrical equipment in the tether. Some prior art like Lavin U.S. Pat. No. 6,325,330 have conductors in the tether while other prior art like Beach et al. U.S. Pat. No. 4,842,221 has a central electrical core surrounded by a strength member. The only patent uncovered with what may be argued as a hollow tether is Schneider U.S. Pat. No. 4,092,827 which provides a duct and tether for transferring collected rain water collected from above the sea and transported to a nearby island. Schneider U.S. Pat. No. 4,092,827 does not replenish lift gas and does not include cables. None of the known prior art has a tether with a hollow core surrounded by a strength member having embedded therein electrical cables and optical telecommunications cables that have at each end novel slip rings to enable lift gas to be purified and replenished from the ground.
It would therefore be advantageous to provide a tethered aerostat system that can continuously cycle and replenish the helium gas without the airship being brought down. It would be advantageous to regulate the quality of the helium and its pressure from the ground without having to vent or dispose of lift gas when the airship is deployed at altitude.